In "The Bow Tie Asymmetry", Howard found his dog Bark Hamill. Mark offers Howard a reward followed by a favor. Howard gets him to perform the Shamy wedding as a surprise to Sheldon displacing Wil Wheaton. Before performing the ceremony, he fills in time taking Star Wars questions that he can't answer. Mark also breaks down during the ceremony because he thinks the vows are so beautiful.
Fun fact: In the episode, Mark Hamill apparently does not know who Wil Wheaton is, but in real life they are very familiar with each other.